It's just that simple, seriously. You can cut it to proper lengths if you want to make one that's not designed directly for 150 MHz (the basic design), but they're so cheap to make it won't hurt to make a few of them, one for each particular band you might be interested in - only takes a few seconds to unscrew one from the PL-259 connector (on the end of some coax, of course). You just need the SO-239 chassis mount jack and wire or coat hangers, some people have used brazing rods (welding stuff) and those work fine too. You don't necessarily have
to add those 450 and 800 MHz parasitic elements but I suppose if it's made according to the design it can't hurt.
I have a 19" main element and 20" ground planes bent to about 45 degrees downward and it picks up everything just fine for me. That Laird 800 MHz may work better than the RS 800 in low signal strength areas, I can't say for sure, so maybe it'll be a good choice for you. Personally I'd say make yourself a 1/4 wave ground plane cut to 855 MHz (the center of the 800 MHz public safety band now because of rebanding) and the main element would be 3.3 inches long - the ground planes make those about 3.5 inches long (base frequency + ~5%) and you should get decent reception on those frequencies in just a few minutes time as long as you have the proper parts and cabling to get it built and attached to your scanner.
Works for me very well and was practically nothing in terms of actual out of pocket cost.
<hint: mine isn't even soldered at all, it's held together just by the screws/nuts and the main element is just inserted into the top of the SO-239 jack. I actually have several elements - one for 150, 450, 855, and 937-ish - that I swap out from time to time just to get a more band-centered antenna for testing purposes - it's not absolutely necessary to do the soldering but it sure can't hurt, even the PL-239 on the coax isn't soldered, it's a screw on with a screw on BNC on the end attached to the pigtail I have plugged into my RTL stick>